Progresses on novel positive electrode materials of rechargeable Li-ion batteries showing safety, sustainability and cost advantages have been recently achieved with the arrival of polyanionic compounds. Pursuing chemical substitutions on the anionic sites our group has synthesized a triplite LiFeSO₄F^1,2 phase with a redox voltage of 3.9 V vs. Li⁺/Li⁰. Another way to modify the redox potential of polyanionic compounds consists in changing the nature of the 3d-metal. Recent DFT calculations have indicated that potentials as high as 5.1 V should be achievable for tavorite LiCuSO₄F.³ This, combined with our recent finding of a 4.7 V redox activity in Li₂CuO(SO₄)₂,⁴ was an impetus to further explore the Cu-based fluorosulfate chemistry which so far counts a sole member, the electrochemically inactive tavorite NaCuSO₄F phase.⁵ Herein we report the synthesis, structure and physical properties of a newly synthesized LiCuSO₄F phase. Interestingly, we show that this new compound crystallizes in a fully-ordered triplite structure with M1 being fully occupied with Cu and M2 with Li (Figure 2a). This is, to the best of our knowledge, the first experimental realization of triplite without any site mixing. This ordered triplite phase, whose stability was confirmed by DFT calculations, was characterized for its electrochemical properties.  

  The Rietveld refinement of synchrotron and neutron diffraction patterns and the structure of LiCuSO₄F is shown in Figure 1.

  The competitive formation of disordered or ordered triplite phase was evaluated through DFT calculations (Figure 2c). We demonstrate that both ordered Cu_M1 and Cu_M2 triplite LiCuSO₄F polymorphs are thermodynamically equivalent, and more stable than either “tavorite” or “disordered triplite” polymorphs. The enthalpy governs the formation of an ordered LiCuSO₄F triplite. This contrasts with the entropy-driven formation of LiFeSO₄F disordered triplite in which all Fe-Li distributions are energetically equivalent.

  This new phase was tested for its electrochemical activity towards Li by assembling, in an argon dry box , LiCuSO₄F/Li Swagelok-type cells using a 1M LiPF₆ solution in 1:1:3 EC : PC : DMC electrolyte. The cells were charged to either 4.9 V or 5.2 V at different rates via a VMP system, but no sign of redox activity could be detected in the voltage composition curves. This means that no Li can be removed from this structure till 5.2V, the voltage at which we found the electrolyte copiously decomposes although DFT calculations shows the feasibility to stabilize an intermediate ordered Li_0.5CuSO₄F phase (Figure 2d). The electrochemical potentials computed for the two consecutive delithiation processes: LiCuSO₄F – 0.5Li  Li_0.5CuSO₄F and Li_0.5CuSO₄F – 0.5Li CuSO₄F are 5.15 V and 5.40 V, respectively. This could not be confirmed experimentally owing to the lack of suitable electrolytes although we cannot eliminate kinetic issues associated to the poor Li ionic conductivity in LiCuSO₄F (not shown here). Thus, the attractiveness of this new phase is limited application-wise.

  Nevertheless bearing in mind the richness of sulfate-/phosphate-based polyanionic electrodes adopting either the tavorite or disordered triplite structures, we believe that such a finding will contribute further in the understanding of these technologically important compounds. 

References

                (1)           Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J.-N.; Barpanda, P.; Tarascon, J.-M. Angewandte Chemie International Edition 2011, 50, 10574.

                (2)           Barpanda, P.; Ati, M.; Melot, B. C.; Rousse, G.; Chotard, J. N.; Doublet, M. L.; Sougrati, M. T.; Corr, S. A.; Jumas, J. C.; Tarascon, J. M. Nat Mater 2011, 10, 772.

                (3)           Mueller, T.; Hautier, G.; Jain, A.; Ceder, G. Chemistry of Materials 2011, 23, 3854.

                (4)           Sun, M.; Rousse, G.; Abakumov, A. M.; Saubanère, M.; Doublet, M.-L.; Rodríguez-Carvajal, J.; Van Tendeloo, G.; Tarascon, J.-M. Chemistry of Materials 2015, 27, 3077.

                (5)           Reynaud, M.; Barpanda, P.; Rousse, G.; Chotard, J.-N.; Melot, B. C.; Recham, N.; Tarascon, J.-M. Solid State Sciences 2012, 14, 15.